Operational amplifiers are widely used in electronic circuits. The operational amplifiers are popular because they have characteristics that are desirable for voltage amplification. The operational amplifier circuits are characterized by high input impedance and low output impedance that allow extremely small input currents, and hence low power, to drive the circuit and relatively stable output for large current flow.
A pair of complimentary metal-oxide semiconductor (CMOS) transistors, including a p-channel MOS and an n-channel MOS, are used to further reduce the input power consumption. The CMOS transistors in field-effect configuration are called MOS field-effect transistors (MOSFET). The MOSFET transistors draw negligible current relative to bipolar transistors because the transistors are controlled at the gate terminal by an electric field rather than by current flow. The electrical current flows from the source terminal to the drain terminal of an n-channel MOSFET transistor when a voltage applied to the gate terminal has a higher value than the voltage applied to the source terminal. In a p-channel MOSFET transistor, the voltage applied to the gate terminal must be lower by a certain value than the voltage applied to the source terminal. This certain value is referred to as a threshold voltage. A threshold voltage of approximately 0.7 to 0.9 volts is required to form a conducting channel between the source and the drain terminals.
A comparator is a circuit whose output indicates which of its two input voltages is larger. The comparator senses small differences between the two input signal voltages and generates a digital output level indicating which input voltage corresponds to the differences.
FIG. 1 shows a comparator circuit 100 with two n-channel MOSFET transistors 106 and 108 configured as a differential amplifier. The p-channel MOSFET transistors 102 and 104 are configured in a cross-coupled pair with positive feedback.
A series of switches S1 through S2, implemented in practice with MOSFET transistors, are used to transition between modes. When the S1 switches are closed, the comparator circuit is in preset mode. The source and the drain terminals of the MOSFET transistors are set to supply voltage level (V.sub.dd) and the transistors are turned off. When the S1 switches are opened and the S2 switch is closed, the circuit enters a differential amplifier comparator mode. Typical V.sub.dd is between 3.0 volts and 5.0 volts.
The circuit current, and thus the power, can be further reduced by lowering the reference voltage (V.sub.dd) as much as possible. Lowering this voltage to 1.0 volt or lower would also allow the amplifier to operate from a single standard battery. However, the reference voltage cannot be reduced below the threshold voltage that is required to form a conducting channel between the source and the drain terminals. In general, the reference voltage cannot be reduced below 1.3 to 1.5 volts without significantly degrading the performance of the transistors.